With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using magnetic recording technology is continuously increasing at an annual rate of approximately 60%. In recent years, an information recording capacity exceeding 320 Gbytes/platter for a magnetic recording medium with a 2.5-inch diameter for use in an HDD or the like has been desired. To fulfill such demands, an information recording density exceeding 500 Gbits per square inch is desired to be achieved.
Important factors to attain a high recording density of a perpendicular magnetic disk include an improvement in TPI (Tracks per Inch) by narrowing a track width, ensuring an electromagnetic conversion characteristic, such as a Signal to Noise Ratio (SNR) or an overwrite characteristic (an OW characteristic) at the time of improving BPI (Bits per Inch), and further ensuring a resistance to thermal fluctuation with the recording bit decreased with the cause described above. Among others, an improvement in SNR under a high recording density condition is important.
In a magnetic layer of a granular structure, which goes mainstream in recent years, a non-magnetic substance having an oxide as a main component is subjected to segregation around magnetic particles having a CoCrPt alloy grown in a columnar shape as a main component to form a grain boundary part. In this structure, since the magnetic particles are separated from each other, noise is reduced, which is effective in achieving a high SNR. An important factor to further improve the SNR is to improve crystal orientation. Co has an hcp structure (a hexagonal close-packed structure), and a c-axis direction (an axial direction of a hexagonal column of a crystal grating) serves as an easy axis of magnetization. Therefore, by orienting the c axis of more crystals in a more perpendicular direction, noise is reduced and also a signal is strengthened, thereby improving the SNR as a synergistic effect.
When a metal film of an hcp structure is formed by sputtering, crystal orientation tends to be improved as the film thickness is thicker. Thus, to improve the crystal orientation of a granular magnetic layer in an initial growth stage, in a conventional technique, Ru, which is a metal of an hcp structure, is used to form a ground layer (also called an intermediate layer), and a granular magnetic film is then formed thereon. Furthermore, a crystalline preliminary ground layer (also called a seed layer) is provided below the Ru ground layer to improve the crystal orientation of the Ru ground layer.
Patent Document 1 describes a structure in which a soft magnetic film configuring a backing layer has an amorphous structure, a ground film (corresponding to a preliminary ground layer in the present invention) is formed of a NiW alloy, and an intermediate film (corresponding to a ground layer in the present invention) is formed of a Ru alloy. According to Patent Document 1, with the ground film being formed of a NiW alloy and the intermediate film being formed of a Ru alloy, excellent productivity and recording and reproduction of high-density information can be achieved.